1. Field of the Invention
The present invention relates to a lacquer-coated wire with a noncircular cross-section having two axes of different lengths and extending perpendicularly to each other. The wire is composed of a metal wire and a lacquer coating surrounding the metal wire, wherein the lacquer coating has insulating and baked enamel properties; the lacquer-coated wire is intended particularly for manufacturing electrical coils. The present invention is also directed to a method of manufacturing the wire.
2. Description of the Related Art
Lacquer-coated wires having a round cross-section for manufacturing electrical coils are known in the art. These wires are composed of copper, aluminum or another metal or a metal alloy and have an inner electrically insulating coating and an outer baked lacquer or baked enamel layer on the inner coating. The first inner lacquer coating, which is composed of polyurethane or polyesteramid, serves for the electrical insulation of the wire. The second outer lacquer layer, also called baked enamel, serves for mechanically connecting the windings and the winding layers. The outer layer is composed of lacquers which make a subsequent softening of the lacquer layer possible. The lacquer layer can be softened either by the influence of heat or by the influence of a chemical solvent. Usually used are lacquers on the basis of polyvinyl butyral or polyamid.
In the following description, the term xe2x80x9croundxe2x80x9d is always understood to mean xe2x80x9ccircularxe2x80x9d, while xe2x80x9cnoncircularxe2x80x9d means that it deviates from the exact circular cross-section. The noncircular cross-sections are essentially oval and rectangular cross-sections, wherein the latter have rounded edges; thus, as defined above, the noncircular cross-sections have two axes of different lengths which extend perpendicularly to each other.
Wires of the above-described type are used for the manufacture of electrical, self-supporting coils. However, these coils with a wire having a round cross-section have the disadvantage that free spaces remain between adjacent wires. These spaces reduce the packing density which reduces the efficiency of the device in which the coil is being used. In addition, the individual wires of such coils only contact each other along very narrow contact lines, so that, also because such coils are subjected to electromechanical forces, the durability of such coils is impaired.
In the attempt to obtain more densely packed coils, it is also known in the art to manufacture the coils of lacquer-coated wires with rectangular cross-sections. Wires of this kind are obtained by deforming the metal core as necessary and subsequently providing the core with the insulating layers.
However, the coated wires with a metal core of rectangular cross-section have the disadvantage that, as a result of the manufacturing method, the coatings are not applied with uniform thickness on the metal core. The reason for this is the lacking symmetry of the rectangular metal wire in connection with the surface stress of the material of the coatings. For this reason, such wires have coatings with convex outer surfaces. When coils are manufactured from these wires, adjacent wires also only contact each other along relatively narrow contact lines, wherein, as is the case in coils manufactured from wires of round cross-section, intermediate spaces remain between the individual wires. Since the lacquer coating and, thus, also the outer baked enamel layer on the wire are applied with different thicknesses over the rectangle surfaces, and since the wires bake together only over relatively narrow contact areas, the adherence between individual wires in the coil is not optimal. This, in turn, significantly reduces the service life and the mechanical self-supporting capability of the coil.
Therefore, it is the primary object of the present invention to provide a noncircular lacquer-coated wire which does not have the disadvantages of the wires described above.
In accordance with the present invention, the wire has a lacquer coating which is composed in the conventional manner of an inner electrically insulating lacquer layer and an outer baked lacquer layer, wherein, in accordance with the present invention, the lacquer layer on the sides to be assigned to the longer cross-sectional axis have a smaller thickness than the sides to be assigned to the shorter cross-sectional axis.
In accordance with an alternative embodiment of the present invention, the metal wire is surrounded by a single lacquer layer which has the insulating properties as well as the baked lacquer properties. As a side to be assigned to a particular cross-sectional axis is the area of the surface which can be considered most closely to extend parallel to that cross-sectional axis.
In accordance with an advantageous further development of the invention, the wire has a cross-section with an axis ratio of at most 3:1, wherein the cross-section is rectangular or, in accordance with an advantageous feature, oval.
The advantage of such a wire according to the present invention is that its configuration makes it possible to manufacture an electrical coil in which adjacent wires are placed closely against each other and bake together over a large surface area. Due to the fact that the lacquer thickness of the lacquer coating of the wire is thicker, preferably, significantly thicker at the sides to be assigned to the shorter cross-sectional axis than at the sides to be assigned to the longer cross-sectional axis, it is possible when manufacturing coils of lacquer-coated wires with particularly an oval cross-section to substantially fill out with lacquer coating material the spaces which result when the wires are arranged next to one another. This is because the coil manufacture takes place by heating so that the lacquer-coating material becomes plastic and particularly the spaces formed in the areas of the sides to be assigned to the shorter cross-sectional axis of the wire can be filled out with lacquer coating material which becomes available to an increased extent at this location. The essentially uniform thickness of the lacquer coating at all sides or surfaces of the wire according to the present invention ensures a close arrangement and baking together of the lacquer-coated wires having rectangular cross-sections when manufacturing coils.
By eliminating the intermediate spaces between adjacent wires, and because the wires bake together closely at all sides, the durability of the electrical coil and, thus, the service life thereof is significantly increased by up to 50%, i.e., very significantly compared to the previously known coils manufactured from a wire having a rectangular cross-section.
The present invention is also directed to a method of manufacturing a wire with a noncircular cross-section which has two axes of different lengths which extend perpendicularly to each other, wherein the wire is composed of a metal wire and a lacquer coating with insulation and baked enamel properties surrounding the metal wire. In accordance with the invention, a lacquer-coated wire having a round cross-section is pressed by means of a rolling procedure into a cross-section having an axis ratio of at most 3:1. In accordance with a preferred embodiment, the wire is pressed so as to have an oval cross-section.
It has surprisingly been found that pressing the coated wire up to this maximum extent is possible without negatively affecting the lacquer coating, while achieving an essentially uniform coating thickness at the individual surfaces of the wire.
The method according to the present invention is preferably carried out in such a way that the lacquer-coated wire having a round cross-section is guided between two appropriately spaced-apart rolls. It is essential that the roll gap is adjusted in such a way that the ratio of the two cross-sectional axes of different lengths and extending perpendicularly to each other of 3:1 of the deformed wire is not exceeded.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.